Covert Channels Through Branch Predictors: a Feasibility Study Dmitry Evtyushkin1 Dmitry Ponomarev1 Nael Abu-Ghazaleh2 1State University of New York at Binghamton Department of Computer Science 2University of California at Riverside Department of Computer Science & Engineering Hardware and Architectural Support for Security and Privacy (HASP) June 14, 2015 Portland, OR, USA in conjunction with ISCA 2015 1

Presentation Outline What is covert channel? How can it be used? 2 What is covert channel? How can it be used? Architectural covert channels Branch Predictor in CPU Constructing covert channel through Branch Predictor Results Optimizations Conclusion 2

Covert Channels Data Sharing Mechanisms: Shared Memory Networking 3 Data Sharing Mechanisms: Shared Memory Networking File system IPC etc... 3

Covert Channels Sharing is not desirable: Violates security policy 4 Sharing is not desirable: Violates security policy Information Flow Control (IFC) schemes TaintDroid (OSDI '10) Applications in different Security Domains Covert Channels: Transfer information through channels not intended for information transfer 4

Covert Channel Usage Example 5 Application is isolated or password data is tainted Are passwords secure? $*$&%*&$(*&&@(#& No IPC No File System Access Password Manager No Network Access (Malicious) Passwords are secure as long as OS enforces security policies $*$&%*&$(*&&@(#& Weather Widget Network Access (Malicious) 5

Covert Channel Usage Example 6 Shared resources can be used to construct covert channels: OS resources File descriptors, free space, etc. Network latencies Power and Thermal effects Architectural resources Caches Functional units Branch predictor Other resources Others 6

Contention-based Architectural Covert Channels 7 mul %r1,%r1 mul %r1,%r1 1 Trojan Spy 1 Multiplication Units CPU 7

Branch Prediction Unit 8 Properties that make covert channel possible: During execution branch predictor accumulates state BP is shared among all processes on core BP is not flushed on context switches Parallel threads in SMT share same BP Branch mispredictions have high cost 8

Branch Predictor Operation 9 9

Constructing Covert Channel 10 Not-taken Taken 1 10

Scheduling Trojan and Spy 11 Single Threaded Scenario: Setting affinity mask to run Trojan and Spy on the same core Quanta Q1 Q2 Q3 Q4 Q5 Thread 0 Multi Threaded Scenario (SMT): Setting affinity mask to run Trojan and Spy on parallel virtual cores Quanta Q1 Q2 Q3 Q4 Q5 Thread 0 Thread 1 11

Branch Code 12 br_taken: push %rbp movl $0x1,-0x8(%rbp) cmpl $0x0,-0x8(%rbp) jne .L2 nop .L2: jne .L3 .L3: ...................... Notes: Trojan and Spy execute large number of branch instructions 500K for Trojan 30K for Spy Trojan and Spy execute the similar code Nop instructions to randomize layout 12

Evaluation Platform Real hardware Intel Core i7-4800MQ CPU (Haswell) 13 Real hardware Intel Core i7-4800MQ CPU (Haswell) Ubuntu 14.04.2 generic Linux kernel version 3.16.0-31 13

Results: Single Thread 14 Single Thread, no interference: 14

Signal vs Noise Single Thread, no interference, cpuburn as noise: 15 Single Thread, no interference, cpuburn as noise: High SNR Stable signal Possibly high capacity Asynchronous signal 15

SMT Results 16 Multi Thread, no interference, cpuburn as noise: 16

Realistic example 17 Single Thread, Browser with YouTube as noise/interference 17

Possible optimizations 18 What we present is only a prototype Can transmit multiple bits at once Can make scheduling faster For more accuracy, can reverse-engineer BP to find optimal number of branch instructions/nops 18

Conclusions 19 Branch predictor can be used as an effective covert channel media Covert channel through BP has desirable properties It should be considered when implementing secure systems free of covert and side channels 19

20 Thank you! 20